Introducing the oxy acetylene system

Go into any workshop in the country, large or
small, and you’ll almost always find an oxy welding kit. There’ll be the black
bottle (oxygen) and the smaller maroon bottle (acetylene), some hoses and a
handpiece. But isn’t an oxy set a bit old-fashioned? After all, these days you
can buy cheap MIGs, plasma cutters – even TIG welders. So what use would have an
oxy welding kit have?

The short answer is: a lot! An oxy-acetylene
welding kit can do all the following:

fusion weld

braze weld

silver solder

heat metal to allow it to be bent and formed

heat metal to allow it to be hardened or
softened

cut metal

That is an incredible list! Why? Well, before we
get into the nitty gritty, here are some real world uses.

Take the first – fusion welding. In brief, that’s
where you melt together metals of the same sort (eg steel), usually with the
addition of small amounts of a filler rod made from the same material as the
metals. Years ago, I fusion welded in this way steel intercooler plumbing. The
same approach works well on exhaust pipes.

Brazing? Again it works well on intercooler
plumbing, and I’ve recently had an oil return line from a turbo cut and then
brazed to suit its new angle. Brazing doesn’t require that the metal is melted –
just heated to a dull red. As a result, there’s less distortion and the process
can be used on very thin metal. Brazing can also be used to join dissimilar
metals – eg copper to steel.

Silver soldering? Again I’ve used this on a turbo
conversion – in that case, on a high pressure oil supply fitting. Silver
soldering (nothing like normal soft soldering!) is like brazing except it uses a
rod containing silver, and is good for very close-fitting parts. It’s also very
strong.

Whenever metals are bent, for example when making
a bracket, hot bending subjects the material to lower stress than cold bending.
The 10 x 4mm steel bar that needs a huge hammering in the vice when it’s cold
becomes the bar easily bent with some gentle taps when it’s hot.

By heating steel to different temperatures
(usually indicated by the colour of the material) and then quenching it in
different baths (eg oil or water), steel can be hardened. Also, materials that
work-harden (eg copper) can be softened by heating and allowing to slowly cool.

Finally, an oxy set can be used to cut metal,
including quite thick steel plate. It doesn’t give the neat edge of water-jet
cutting, but the steel plate of the turbo exhaust manifold I built a few years
ago was oxy cut, the edge being then cleaned-up with a file and
grinder.

So an oxy-acetylene kit allows you to do nearly
everything required when welding, brazing, softening/hardening or cutting of
metals is needed. However, compared with other welding and cutting techniques,
it’s often slower – which is a downside in production work. But for one-offs and
home workshop use, the slow pace of the work allows far better user control.

For example, I have recently been brazing together
some very thin wall (0.9mm) high tensile (chrome moly) steel tubing. The brazing
rods being used are nickel bronze – a very strong brazing material. Normally,
welding such thin wall steel tube would be very difficult – whether by MIG or
TIG. (These tubes are quite a lot thinner than exhaust tube, for example.) But
brazing these tubes with the oxy kit is child’s play.

Why? Well, I can braze each joint without fear of
melting away the parent material, I can add or remove heat as easily as applying
or removing the flame (and of course, also set the starting point by the
appropriate selection of flame and tip size), and I can tack the joints and then
came back later and seamlessly extend them to full welds. I can also make nuts
captive by brazing them in place, and I can easily do tricky things like brazing
a disc flush over the end of the tube. And, if I make a mistake, I can very
easily ‘un-tack’ the braze. Now brazing isn’t as strong as MIG or TIG or fusion
welding, but its versatility and ease of control make it unbeatable for my skill
level in this application.

The other point this leads to is that an
oxy-acetylene kit is best suited for smaller, fiddly jobs. If I built a
trailer, I’d use electric welding – arc or MIG. You could certainly do it with
an oxy, but you’d be there a long time. In the same way, material thicker than
about 5mm is usually electrically welded. Finally, while brazing rods are
available for aluminium, the success or otherwise of this depends a lot on the
exact make-up of the aluminium (something normally unknown!). So an oxy kit is
not normally used to weld aluminium.

OK, enough of the prelude: let’s look at the
equipment.

Equipment

As the name suggests, oxy acetylene welding uses
two gases – acetylene and oxygen. The acetylene is the fuel and the oxygen helps
achieve the very high 3100 degrees C flame temperature required.

Acetylene cylinders are filled with a
porous mass which is saturated with acetone. The acetylene dissolves in the
acetone much like carbon dioxide is dissolved in the liquid in a soft drink
bottle: when the pressure is lowered, the acetylene bubbles out of the acetone.
Acetylene cylinders are shorter than oxygen cylinders, are painted maroon (deep
red) in colour, and use a left-hand (ie reversed) thread to prevent inadvertent
coupling of oxygen fittings.

Oxygen cylinders are taller than acetylene
cylinders. They are painted black and use a conventional right-hand thread. The
oxygen is compressed to a larger degree than the acetylene and so the cylinders
use heavy walls and are in turn heavy.

Oxygen fittings should be kept completely free of
grease or oil; should these contaminants come in contact with the oxygen, an
explosion can occur.

Each cylinder is equipped with a shut-off valve –
like a tap in your bathroom, rotate clockwise to close. Fittings connected to
both types of cylinder should be specifically design for the application. For
example, copper fittings should not be connected to acetylene cylinders as the
copper reacts with the acetylene, creating highly explosive copper acetylide.

Each cylinder uses a pressureregulator. These are used to reduce the pressure from the massive bottle
pressure to that which is suitable for use. Each regulator has two gauges. One
shows the bottle pressure (so giving an indication of how much gas is left in
the cylinder) and the other shows the set pressure of the gas being fed to the
handpiece. Typically, full acetylene and oxygen cylinders will have pressures of
1800 kPa and 17500 kPa respectively, while the gas pressures for normal fusion
welding or brazing are set at 50 kPa.

The name given to the combination of the
handpiece, control valves and welding tip is blowpipe.

The control valves on the blowpipe allow
user-variation of the flow of the two gases. These controls are very important
as they allow two things: (a) setting of the flame intensity, and (b) setting of
the ratio of oxygen to acetylene.

The handpiece is not only the bit you hold
but also contains two tubes that feed the gases to the mixer. As its name
suggests, the mixer brings the two gases together. Furthermore, the mixer
contains some safety devices preventing burning-back of gases through the hoses.

The welding tip is the curved nozzle
through which the mixed gases pass. Tips are available in different sizes,
varying both with both physical size and also the diameter of the orifice at the
end. (Tips with small orifices are physically smaller overall.) Welding tips are
easily swapped as required – they simply unscrew from the blowpipe.

Hoses are used to connect the regulators to
the blowpipe. These hoses are colour-coded – blue for oxygen and red for
acetylene. Finally, flashback arrestors are sometimes fitted to the
blowpipe. These lessen the chance of the flame burning its way back towards the
cylinders.

Setting Up

Unlike arc welding that we covered in Part 1 of
this series, an oxy acetylene kit has some potentially major safety issues. The
bottled gases are under very high pressures, are extremely inflammable when
mixed, and even when unmixed are hazardous. A hose leaking at a fitting is
clearly very dangerous. (Note: hose and regulator fittings should be done up
with an appropriate spanner. But don’t go mad with tightening torque – a nip-up
is sufficient.)

The following process should be followed when
initially setting up the gear:

Ensure your hands are free of grease and
oil.

After ensuring that there is no source of
ignition in the vicinity (including gas hot water heater pilot flames!), momentarily open the cylinder valve to blast any foreign bodies from the
outlets.

Make sure the regulator knob is undone (rotated
anti-clockwise until loose) and then attach the regular to the cylinder.
(Remember the threads are different direction, depending on the
cylinder!)

Open the cylinder valve slowly. The high pressure
gauge will show full cylinder pressure.

Check for leakage by closing the cylinder
valve and checking that the indicated pressure does not drop.

Do the same with the other cylinder.

With the regulators safely on the cylinders, you
have now completed the first step – you have low pressure gas available from the
cylinders. Now to get that gas to the blowpipe.

Connect the appropriate coloured hoses to the
appropriate cylinders – blue to oxygen, red to acetylene.

Purge the air from the hoses by momentarily
screwing down the regulator knobs. You’ll be able to hear gas flowing from the
end of the hoses. Firstly, ensure that there is no source of ignition in the
vicinity.

If you are using flashback arrestors (some kits
don’t include them), connect them to the ends of the hoses. The blue hose goes
to the oxygen arrestor and the red hose to the acetylene arrestor.

Connect the hoses to the blowpipe. If you are
using an off-the-shelf kit, the hoses will be staggered in length to match the
offset of the blowpipe fittings.

OK, so now you have the complete system set up and
ready for working. Now for a very important test.

Close the blowpipe taps and screw in the regulator
knobs until each associated pressure gauge indicates 50 kPa (7 psi).

Now close the bottle valves and check that the 50
kPa gauge readings don’t slowly drop. If they drop in reading, there is a
leak!

If there is no leak, open the bottle valves
again.

If there is a leak, check all fittings and
hoses.

Now we’re getting close to being able to
weld....

Select the appropriate welding tip and screw it
into the blowpipe. This table shows how to go about selecting the tip.

Plate

(mm)

Tip Size

Acetylene (kPa)

Oxygen (kPa)

0.8

6

50

50

1

8

50

50

1.6

10

50

50

2.5

12

50

50

3.5

15

50

50

5

20

50

50

8

26

50

50

Finally, check that delivery pressure is
maintained (ie 50 kPa) when the blowpipe taps are opened. Again ensure that
there is no source of ignition in the vicinity. If the delivery pressure
drops, open the main bottle valves further.

So that’s the welding gear set up and safe.

Shutting Down

We haven’t even started welding but it’s best if
at this point we cover shutting the system down.

Close the main cylinder valves

Unscrew the regulator handles

Open the blowpipe valves and release the gas in
the hoses. Again, ensure that there is no source of ignition in the
vicinity.

Close the blowpipe valves

Next week: neutral, oxidising and carburising
flames, and brazing.

Cylinder
Hire

Gas
welding kits are widely available. They contain the regulators, hoses, blowpipe,
welding tips and so on. However, you’ll find, in Australia at least, that you
have to hire the cylinders. So not only do you pay for the gas, you also pay to
have the cylinders sitting there, perhaps doing nothing for a long time.

I
found that cylinder hire costs varied a great deal: when I enquired by phone a
few years ago, the cost was outrageous. But when I walked into the local BOC
outlet, wearing big steel-capped work-boots and a dirty AutoSpeed shirt, and told them I had
an Australian Business number (ABN), the cylinder hire costs were very
reasonable....

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